1. Field
Example embodiments relate to an engine. More particularly, example embodiments relate to a compressed natural gas (CNG) engine.
2. Description of the Related Art
Most of engines may have an overlap at which an intake valve and an exhaust valve may be simultaneously opened. Generally, the intake valve may be opened before a top dead point of a piston and closed after a bottom dead point of the piston. The exhaust valve may be opened before the top dead point and closed after the top dead point.
Particularly, when the intake valve may be so quickly closed, an amount of an intake air may be decreased to reduce a volumetric efficiency. In contrast, the intake valve may be closed lately, a mixture in a cylinder may flow backwardly toward an intake port to reduce the volumetric efficiency. Further, when the exhaust valve may be so quickly closed, an exhaust gas may not be sufficiently exhausted. In contrast, the exhaust valve may be closed late, the exhaust gas may flow backwardly from an exhaust port to the cylinder to reduce the volumetric efficiency. The mixture, which may not be combusted in the cylinder, may be exhausted through the exhaust port into decrease fuel efficiency, thereby causing fail of compliance with emission regulations with respect to a CH4 gas.
In a compressed natural gas (CNG) engine, a fuel and an air may be mixed from an intake port to form a mixture. The mixture may be introduced into a cylinder. Therefore, a performance of the CNG engine may be dependent upon opening times and closing times of an intake valve and an exhaust valve.